Temperature sensitive mutants of Simian Virus 40 have been tested at the permissive and nonpermissive temperatures to determine which viral genes are involved in the stimulation of the 0.2 RF cellular thymidine kinase activity after viral infection of mouse cells. SV40 wild type virus and all ts mutants (A,B,BC,C,D) at the permissive temperature stimulated this enzyme activity. At the nonpermissive temperature (41 C) SV40 tsA and tsD mutants failed to stimulate the cellular thymidine kinase activity, while SV40 tsB, BC and C mutants did stimulate this enzyme activity. Equine Herpes virus Ky A and D strains have been shown to differ with regard to their ability to produce a thymidine kinase activity after viral infection. The EHV-1 Kyd strain produces a thymidine kinase activity distinct from both cellular TK activities in electrophoretic mobility and sensitivity to several inhibitors. The EHV-1 KyA strain does not produce this novel thymidine kinase activity. The EHV-1 KyA and D viruses are both neutralized by anti EHV-1 antisera to a similar extent with the same kinetics. BIBLIOGRAPHIC REFERENCE: E.H. Postel and A.J. Levine: The Requirement of Simian Virus 40 Gene A Product for the Stimulation of Cellular Thymidine Kinase Activity after Viral Infection, Virology, 1976, in press.